bin_prot 0.16.0 doesn't like trivial cases in mutually recursive type definitions

The following snippet fails to compile:

```ocaml
open Bin_prot.Std

type ab =
| A of a
| B of int

and a = ab [@@deriving bin_io]
```

with error message

```
7 | and a = ab [@@deriving bin_io]
^^
Error: This kind of expression is not allowed as right-hand side of `let rec'
```

It builds if we remove `[@@deriving bin_io]` or if we replace `bin_io` with `show`.

Tested with `bin_prot 0.16.0`, can't migrate to `0.17.x` atm because of dependencies that are not compatible with OCaml 5.x.

Reproduce the mutually recursive type definition from the issue with bin_prot 0.16.0 and the bin_io deriver, then trace how this case is handled during compilation. Done means the snippet compiles successfully and the trivial mutually recursive case has regression coverage.

Written by the indexing model from the issue text.